Peroxynitrite is formed when nitric oxide reacts rapidly with superoxide anion radical. The peroxynitrite anion is stable; however, its conjugate acid, with a pKa of 6.7, and its CO2 adduct, ONOOCO2-, are strong oxidants capable of oxidizing sulfhydryl and thioethers and nitrating aromatic compounds. Our previous study revealed that peroxynitrite can nitrate tyrosine residues of cdc2(6-20)NH2, the synthetic peptide which is a good substrate of lck protein tyrosine kinase. When Tyr-15, or both Tyr-15 and 19, in cdc2(6-20)NH2 were replaced by nitrotyrosine, both synthetic peptides failed to be phosphorylated by lck kinase. The present report shows that the nitration reaction is enhanced by either Fe(III)EDTA or Cu(II)-Zn(II)-superoxide dismutase (Cu,Zn-SOD). The kinetic data are consistent with the view that reaction of Fe(II)EDTA or Cu,Zn-SOD with the cis form of peroxynitrite yields complexes in which peroxynitrite decomposes more slowly to form NO2+, the nitrating agent. Thus, the nitration efficiency of peroxynitrite is enhanced. Phosphoinositide-specific phospholipase C type gamma (PLC-gamma) and other src homology 2 (SH2) domains have been shown to bind specifically to tyrosine-phosphorylated cellular proteins. By using surface plasmon resonance to measure the binding and kinetics of the SH2 domains of PLC- gamma and its SH2 recognizable sites, our present study shows that specific amino acid sequences of the synthetic phosphopeptides of ENAEY(PO3H)LDLDC and CGDNDY(PO3H)IIPL can bind the SH2 domains of PLC- gamma. However, peroxynitrite-mediated tyrosine nitration of these specific synthetic peptides failed to bind SH2 domains. This result indicates that peroxynitrite-mediated nitration of tyrosine residues of SH2 recognizable sites failed to bind SH2 domains of PLC-gamma.